A methanol carbonylation process is known as a acetic acid synthesizing process that is suitable for acetic acid industrial production. With this synthesizing process, starting materials methanol and carbon monoxide are reacted with each other in the presence of a predetermined catalyst to form acetic acid.
An acetic acid production plant for use in acetic acid production using the methanol carbonylation process includes two or more units such as a reactor, a flash evaporator, a low-boiling component removing column, and a dehydration column. In the acetic acid production plant as mentioned above, acetic acid is produced typically through processes in the individual units as follows. In the reactor, acetic acid is continuously formed from starting materials methanol and carbon monoxide by a methanol carbonylation reaction. In the flash evaporator, a reaction liquid from the reactor, where the reaction liquid contains acetic acid formed in the reactor, is subjected to a so-called flash evaporation treatment to extract vapor of crude acetic acid from the reaction liquid. In the low-boiling component removing column, the crude acetic acid is subjected to distillation, and a liquid acetic acid stream enriched with acetic acid is drawn out of the low-boiling component removing column. The distillation is performed mainly for removing low-boiling components from the crude acetic acid, where the low-boiling components have lower boiling points as compared with acetic acid. In the dehydration column, the acetic acid stream is subjected to distillation mainly for removing water from the acetic acid stream, and a liquid acetic acid stream further enriched with acetic acid is drawn out of the dehydration column.
The methanol carbonylation process may employ iodide as a promoter for assisting the action of a catalyst to be used. Iodine, when used, forms hydrogen iodide as a by-product in the reactor. The hydrogen iodide, with main product acetic acid and other substances, passes through the units in the acetic acid production plant, acts as a strong acid, and causes corrosion of the acetic acid production plant. Techniques for decreasing the hydrogen iodide concentration in the low-boiling component removing column can be found typically in PCT International Publication Number WO 2013/137236 (PTL 1). Techniques for decreasing the hydrogen iodide concentration in the dehydration column can be found typically in PCT International Publication Number WO 2012/086386 (PTL 2).